Spectrum and nature of surface states

With the help of an approximative E(k) relation in the forbidden gap N(E) distributions for localized states are calculated. A potential fluctuation model gives energy distributions of states in the gap which exhibit features of surface state spectra or spectra of disordered systems. Binding and nonbinding states are coupled to pairs by the E(k) relation and characteristic properties of surface and interface state spectra like symmetry, acceptordonor character, minimum of the N(E) distribution may be described. Dangling bonds of preferential 111 character are the proper chemical picture. Approximative energy calculations show a preferential energy range for isolated interface states at a distance of 0.1–0.2 eV distance from the band edges. Comparison with experimental termspectra shows a good agreement with the treated model. Common characteristics of clean, ordinary and oxidized surfaces and the metal semiconductor contact are revealed.     

Strength distributions and statistical spectroscopy II. Shell-model comparisons

The theory of the preceding paper is applied to a number of M1, E2, and E4 electro-magnetic excitations in the (ds)⁶ space. Comparisons are made in detail with shell-model results for the pth energy-weighted sum rules, with p = 0, 1, 2 and starting states spanning the entire spectra, as well as with the exothermic-endothermic decomposition of the non-energy weighted sum rule, and the RMS fluctuations in, and correlations between, the sum-rule quantities. Further comparisons are made for the strength distributions themselves. In all cases the agreements are good, for the sum rules remarkably so, so that the statistical theory describes very well the essential features of the strength distribution. The only (partial) exception is with the usual low-lying quadrupole collectivity found microscopically for two of the starting states (for which most of the strength goes to a single final state) and predicted, though not in such detail, via a statistical calculation of the effective number of final states available for the quadrupole transitions. We are seeing here a real coexistence of collective and statistical phenomena. At higher excitations, where concentration of much of the strength into a single state is not to be expected, all the essential features should be statistically describable. As a result of the comparisons, we expect that the statistical theory, supplemented by further methods for the evaluation of the necessary input traces, should give an almost complete account of the essential features of the strength distributions, even in model spaces of arbitrarily large dimensionality.     

Photoionization cross sections of Fe XXI

Total and partial photoionization cross sections for (Fe XXI+hν→Fe XXII+e) are presented for the ground and excited bound states with n?10 and l?9. Fe XXI is prevalent in high-temperature astrophysical plasmas as well as in photoionized plasmas excited by hard X-rays. Results are reported for the first time for the high-energy photoionization with core excitations to n=2,3 states. Details of photoionization, especially the high-energy features that often dominate considerably over the low energy ones, are illustrated. These prominent features will affect the photoionization and the recombination rates in high-temperature plasmas. Calculations are carried out in the close coupling (CC) approximation using the R-matrix method. A large CC wavefunction expansion for Fe XXII which includes the ground and 28 excited core states from n=2 and 3 complexes and spans over a wide energy range is used. A total of 835 discrete bound states of Fe XXI in the singlet, triplet, and quintet symmetries are obtained. Total photoionization cross sections, σ_PI(nLS), for ionization into all 29 states are presented for all 835 final bound states and partial photoionization cross sections, σ_PI(g,nLS), for ionization into the ground ²P⁰ state of the core are presented for 685 states. While the n=2 core excitations are at relatively lower energy range (within 15 Ry from the ionization threshold), the n=3 excitations lie at considerably higher energy, 73 Ry and above, yet introduce resonant features and enhancements more prominent than those of n=2 states. Larger numbers of resonances are formed due to Rydberg series of autoionizing states converging on to the 29 core states. However, most noticeable structures are formed in the excited state cross sections by the photoexcitation-of-core (PEC) resonances in the photon energy range of 73-82 Ry. All these high-energy features are absent in the currently available results. The present results should enable more accurate modeling of the emission spectrum of highly excited plasma from the optical to far-ultraviolet region.     

Formation of intercalate-like systems of graphite-ytterbium monolayers on the Ni(111) surface

The ytterbium intercalation under a graphite monolayer formed on the Ni(111) surface has been studied by Auger electron and angle-resolved photoelectron spectroscopy. The features of the electronic structure of the intercalate-like thin-film compound formed in this process are analyzed. It is shown that the energy shift of the π and σ states in the valence band toward higher binding energies (by ～2 and ～1 eV, respectively) can be described in terms of hybridization of the carbon π states in the graphite monolayer with the d states of the underlying metal.     

C 1s NEXAFS study of rare-earth’s (La,Eu)-graphite intercalation compounds

Electronic structure of unoccupied states of Eu- and thin surface layer of La-intercalation compounds was studied by light polarization dependent NEXAFS at the C 1s threshold in a bulk sensitive (E_kin=1–2 eV) and a more surface sensitive (E_kin=265 eV) partial electron yield mode. It was shown that the C 1s spectra in both cases are mainly characterized by the π*- and σ*-symmetry graphite-derived features. For both systems the π*-derived peak was found at similar energies of exciting photons as for pristine graphite. A decrease of relative intensity of the π*-originated structure in intercalation compounds can be understood by partial occupation of the π*-derived states upon intercalation due to a charge transfer from rare-earth (RE) atoms. NEXAFS features found on both sides of the π* response may be related to pd hybrids forming as a result of chemical interaction between RE atoms and graphite layers.     

First-principles study of the sulfur K and L2,3 edges of transition metal disulfide monolayers,MS2 (M=Mo,W and Re)

By means of the energy loss near edge structure (ELNES) analysis, the electronic structures of layered transition metal disulfides were studied. In the framework of full potential linearized augmented plane wave method, ELNES spectra of sulfur K and L_2,3 edges of layered MoS₂, WS₂ and ReS₂ have been calculated at magic angle conditions, and compared with those of bulks and the only existing experimental fine structure. Compared to the bulks, the energy differences between the main peaks in sulfur K and L_2,3 edges of monolayers decrease due to the slightly larger bond lengths that it can be used as a fingerprint for monolayers. Sulfur K edges in monolayers include some main features originated from electron transition to p_z (π) and p_x+p_y (σ) states and their hybridization. The overall dispersions of the sulfur L_2,3 edges in all cases are similar to the d-symmetry density of states. The first two features in L_2,3 edge of bulks and monolayers can be attributed to electron transition of sulfur 2p to the both unoccupied 3s-like states of sulfur and 4d states of transition metal atoms. Due to the considerable amount of s states at the energy position of a shoulder like structure in L_2,3 edge of both bulks and monolayers, these structures can be assigned to the sulfur 2p electron transition to unoccupied sulfur 3s states. The other features at higher energies are due to the transition of sulfur 2p electrons to the d-symmetry states of sulfur. In addition, due to the considerable energy band gaps, it seems that the use of core–hole approximation is essential for accurate reproduction of ELNES features of transition metal disulfides.     

Electron energy loss spectrum of graphane from first-principles calculations

In this study, the energy loss near edge structure (ELNES) of carbon atoms in chair and tricycle conformers of hydrogenated graphene, namely ‘graphane’, has been calculated in the density functional theory using FP-LAPW method, and then, it has been compared with that of graphite and graphene. Using ELNES from chair conformer, the carbon K-edge was found to have a few main features including electron transition from 1s orbital of carbon atom to π*, σ*, and a hybridization of these two states. The first feature in tricycle conformer, however, has contributions of both π* and σ* states. The comparison of ELNES and the unoccupied density of states in each structure also justifies this. The energy difference between π* and σ* features of graphane conformers was decreased relative to it in graphite and graphene. Since the inclusion of core-holes and super-cells is essential for accurate reproduction of features in graphite and graphene, it may be essential as well for the ELNES spectra of graphane conformers.     

Isospin mixing in the electric-dipole He(γ, n) and He(γ, p) reactions

Possible isospin mixing of the J^π = 1⁻ excited states of ⁴He leading to sizable differences in the ⁴He(γ, n)³He and⁴He(γ, p)³H cross sections in the giant-resonance region is examined in a bound-state approximation. Results indicate that the general features of certain experimental data can be accounted for by the presence of Coulomb mixing of the (S = 1, T = 0) and (S = 1, T = 1) states in addition to the usual spin-orbit mixing of the (S = 0, T = 1) and (S = 1, T = 1) states.     

Frozen entanglement and quantum correlations of one-parameter qubit−qutrit states under classical noise effects

We provide a detailed analysis of the dynamics of entanglement and quantum correlations for one-parameter qubit-qutrit states under independent or common classical noises influence. Namely the static noise, the Ornstein-Uhlenbeck (OU) noise and the random telegraph noise. Independently of the intrinsic features of the noises, entanglement measured by negativity and quantum correlations measured by measured-induced disturbance (MID) vanish after a finite time under the effects of independent noise environments. In a common environment setup, we show the existence of specific and very important features of perfect insulation of the systems quantum properties from noise effects, for suitable range of the entanglement parameter. We refer these phenomena to as frozen entanglement and frozen quantum correlations. The dichotomy between entanglement (separability) and quantum correlations is strengthened by our results, with the robustness of MID over entanglement and existence of separable qubit-qutrit states with non-zero quantum correlations.     

Features of the Quenching of the Triplet States of Water-Soluble Porphyrins by Molecular Oxygen

Using the methods of time-resolved absorption spectroscopy, we have investigated the features of quenching, by molecular oxygen, of the excited triplet states of water-soluble 5,10,15,20-tetrakis-(4-N-methylpyridyl)-porphyrin (H₂TMPyP) and 5,10,15,20-tetrakis-(4-sulfonatophenyl)-porphyrin (H₂TSPP) in water–ethanol solutions. It has been revealed that for both compounds the rate constant of quenching of the triplet states increases with increasing viscosity of the medium. Quenching of the excited triplet states of the dissociated (in water) and undissociated (in ethanol) forms of water-soluble porphyrins occurs with a different efficiency, and the rate constant of quenching the triplet states by molecular oxygen k _T thereby is higher for the dissociated form. It has been shown by means of mathematical modeling that the experimental results obtained can be described in terms of the change in the rate constants of intracomplex transitions in the porphyrin–oxygen collisional complex at varied solution viscosity and their difference for the dissociated and undissociated forms of water-soluble porphyrin.     

The valence electronic structure of conducting pyrrole polymers: Evidence against a linear metallic chain picture

A UPS study of various conducting polypyrrole films is presented. Most of the valence band features can be explained by states derived from the orbitals of the pyrrole monomer and the associated anion molecules. In close vicinity of the Fermi energy, a density of states is observed which decreases linearly towards E_F. The corresponding states are introduced by oligomer formation. The π-electronic density at E_F is reduced by at least two orders of magnitude compared to ordinary sp-metals. The UPS spectra are consistent with short conjugation lengths and large amounts of disorder, but the corresponding defect states can not directly be observed.     

Vacancy induced changes in the electronic structure of titanium carbide—I. Band structure and density of states

Results of a self-consistent “Augmented Plane Wave” (APW) band-structure calculation are presented for substoichiometric titanium carbide with 25% vacancies on the carbon sublattice sites (TiC_{0 75}) assuming a model structure with ordered vacancies Comparison with an earlier APW study on stoichiometric TiC reveals that the carbon vacancies induce two pronounced peaks in the density of states (DOS), 0.4 eV below and 0.8 eV above the Fermi energy E_γ, thus forming electronic states in a region where the DOS for stoichiometric TiC exhibits a minimum So-called “vacancy states” with an important amount of charge on the vacancy site are found to be derived from Ti 3d states extending into the vacancy muffin tin sphere An angular momentum decomposition with respect to the center of the vacancy muffin tin sphere shows that the s character predominates for the occupied and the p character for the unoccupied “vacancy states” The theoretical findings explain features near E_γ, observed in recently published X-ray emission spectra Furthermore, we find a slight increase of electronic charge in the carbon muffin tin spheres as compared with stoichiometnc TiC.     

First-principles study of photoconductivity in BaTiO3 with oxygen vacancies

The photoconductivity of BaTiO_2.5 with oxygen vacancy has been studied by the linear muffin-tin orbital method in the atomic sphere approximation (LMTO-ASA). The ground-state structure of BaTiO_2.5 is obtained by minimization of the total energy. The partial densities of states show that the occupied states at the bottom of the conduction band have primarily Ti d orbital character. The photoconductivity shows that two novel features, in the low energy side, can be attributed to the intraband transition of free electronic carriers in the vicinity of the Fermi level and the interband transition of the Ti 3d(yz) related band states, to the Ti 3d(xy,xz) related band states, respectively. In addition, it is also found that the anisotropy of photoconductivity is enhanced because of the introduction of oxygen vacancy. The system can show the conductive behavior of electronic carriers, which is qualitatively in agreement with a recent experimental finding.     

Electron transport in granular amorphous silicon dioxide films with ferromagnetic nanoparticles placed in a magnetic field

Electron transport in amorphous silicon dioxide films with embedded nanoparticles (Co, Nb, Ta) was studied. The mean number of localized states in the interparticle tunneling channel was derived from the temperature dependence of conductivity for various grain concentrations under the assumption of the electron transport being governed by resonance tunneling in a chain of localized states between grains. To confirm the assumption of the inelastic character of tunneling, the dependences of the magnetoresistance on grain concentration, temperature, and magnetic field were studied. Accepting the single-orbital model, where the intergrain tunneling magnetoresistance is determined by s-s tunneling, it was found that the existence of weakly split localized states in the tunneling channel results in a lack of magnetoresistance saturation in strong magnetic fields. The combined effect of a decrease in the s-s tunneling coefficient and of growth in the probability of inelastic electron spin scattering with increasing length of the chain of localized states between particles in which the electron is tunneling accounts for the characteristic temperature-concentration dependences of the magnetoresistance. The experimental observation of these features provides an argument for the electron transport in a-SiO₂(Co,Nb,Ta) structures being governed by inelastic resonance tunneling through intergrain localized states.     

Transport through a quantum dot coupled to two Majorana bound states

We investigate electron transport inside a ring system composed of a quantum dot (QD) coupled to two Majorana bound states confined at the ends of a one-dimensional topological superconductor nanowire. By tuning the magnetic flux threading through the ring, the model system we consider can be switched into states with or without zero-energy modes when the nanowire is in its topological phase. We find that the Fano profile in the conductance spectrum due to the interference between bound and continuum states exhibits markedly different features for these two different situations, which consequently can be used to detect the Majorana zero-energy mode. Most interestingly, as a periodic function of magnetic flux, the conductance shows 2π periodicity when the two Majorana bound states are nonoverlapping (as in an infinitely long nanowire) but displays 4π periodicity when the overlapping becomes nonzero (as in a finite length nanowire). We map the model system into a QD–Kitaev ring in the Majorana fermion representation and affirm these different characteristics by checking the energy spectrum.     

Theory of valence mixing for rare-earth compounds

A theory for the mixed valence state of rare-earth compounds is presented. It includes the following features: (1) two types of electronic states-localized, highly correlated states and itinerant, non-correlated states; (2) a very strong Coulomb repulsion between localized states in the same site; (3) a Coulomb interaction between localized and itinerant states which drives the phase transition; and (4) hybridization between localized and itinerant states which produces the mixed valence state. It is shown that this model produces (a) at T = 0, a variation in the number of localized electrons which may vary in a smooth or in a discontinuous fashion as a function of pressure or alloying; (b) transitions at finite temperature which terminate in a classical critical point. Qualitative agreement with experiment is an encouraging feature of the model.     

Unoccupied surface and conduction band states on Ge (111) from energy loss spectroscopy

Unoccupied surface states are observed on Ge (111) by measuring 3d-core level to conduction-band transitions using electron energy loss spectroscopy. These experiments also yield values for conduction band density-of-states features.     

Excitation of lowest electronic states of thymine by slow electrons

Excitation of lowest electronic states of the thymine molecules in the gas phase is studied by elec- tron energy loss spectroscopy. In addition to dipole-allowed transitions to singlet states, transitions to the lowest triplet states were observed. The low-energy features of the spectrum at 3.66 and 4.61 eV are identified with the excitation of the first triplet states 1₃ A′ (π → π*) and 1₃ A″ (n → π*). The higher-lying features at 4.96, 5.75, 6.17, and 7.35 eV are assigned mainly to the excitation of the π → π* transitions to the singlet states of the molecule. The excitation dynamics of the lowest states is studied. It is found that the first triplet state 1³ A′(π → π*) is most efficiently excited at a residual energy close to zero, while the singlet 2₁ A′(π → π*) state is excited with almost identical efficiency at different residual energies.     

Spontaneous breaking of chiral symmetry, and eventually of parity, in a σ-model with two Mexican hats

A σ-model with two linked Mexican hats is discussed. This scenario could be realized in low-energy QCD when the ground state and the first excited (pseudo)scalar mesons are included, and where not only in the subspace of the ground states, but also in that of the first excited states, a Mexican hat potential is present. This possibility can change some basic features of a low-energy hadronic theory of QCD. It is also shown that spontaneous breaking of parity can occur in the vacuum for some parameter choice of the model.